1. Field of the Invention
The present invention relates to a film forming apparatus and a film forming method used for producing a light-emitting element including a film (hereinafter, referred to as an “organic compound layer”) containing an organic compound that emits light by the application with an electric field, an anode, and a cathode. In particular, the present invention relates to production of a light-emitting element that is more unlikely to be degraded and has a longer life as compared with the conventional example.
2. Description of the Related Art
In recent years, a light-emitting apparatus having an organic light-emitting element as a self-luminous element is being actively studied. In particular, a light-emitting apparatus using an organic compound as a light-emitting material is drawing attention. Such a light-emitting apparatus is called an organic electroluminescence display (OELD) or an organic light-emitting diode (OLED).
A light-emitting element includes a layer (hereinafter, referred to as an “organic compound layer”) containing an organic compound in which electroluminescence occurs by the application with an electric field, an anode, and a cathode. Luminescence in an organic compound includes fluorescence that occurs when an energy level returns from a singlet excited state to a ground state and phosphorescence that occurs when an energy level returns from a triplet excited state to a ground state. A light-emitting apparatus produced by the film forming apparatus and film forming method of the present invention is applicable to both the case using fluorescence and the case using phosphorescence.
Unlike a liquid crystal display apparatus, a light-emitting apparatus is of a self-luminous type, so that it has no problem of a viewing angle. More specifically, a light-emitting apparatus is more suitable as a display used outside, compared with a liquid crystal display. Thus, the use of a light-emitting apparatus in various forms has been proposed.
A light-emitting element has a structure in which an organic compound layer is interposed between a pair of electrodes, and the organic compound layer generally has a laminate structure. A typical example includes a laminate structure: “hole transport layer/light-emitting layer/electron transport layer” proposed by Tang et al. of Eastman Kodak Co. This structure has a very high light-emitting efficiency, and most of the light-emitting apparatuses that are being developed have this structure.
The following structures may also be used: “hole injection layer/hole transport layer/light-emitting layer/electron transport layer, which are stacked in this order on an anode”, and “hole injection layer/hole transport layer/light-emitting layer/electron transport layer/electron injection layer, which are stacked in this order on an anode”. A light-emitting layer may be doped with a fluorescent coloring matter or the like. Furthermore, these layers may be formed only of low-molecular weight materials, or may be formed only of high-molecular weight materials.
In the present specification, all the layers provided between a cathode and an anode are collectively called an organic compound layer. Therefore, the above-mentioned hole injection layer, hole transport layer, light-emitting layer, electron transport layer, and electron injection layer are all included in the organic compound layer. Furthermore, an element composed of a cathode, an organic compound layer, and an anode is called a light-emitting element. The light-emitting element has two kinds: a simple matrix system and an active matrix system. According to the simple matrix system, an organic compound layer is formed between two kinds of stripe-shaped electrodes provided so as to be orthogonal to each other. According to the active matrix system, an organic compound layer is formed between pixel electrodes arranged in a matrix so as to be connected to TFTs and a counter electrode.
The most serious problem of practical use of these light-emitting elements lies in that the element's life is insufficient. Degradation of an element is exhibited in such a manner that a non-light emitting region (dark spot) spreads along with light-emission for a long period of time. This is caused by degradation of an organic compound layer.
An organic compound constituting an organic compound layer is degraded by impurities such as oxygen and water. Furthermore, inclusion of impurities of an organic material and an inorganic material in an organic compound may cause an organic compound layer to be degraded.
Conventionally, when a film is formed by evaporation, an organic compound that is an evaporation material is used as it is. However, there is a problem that an impurity is mixed in an organic compound by the time when the organic compound is provided at an evaporation source. Furthermore, in order to prolong the life of an element, an organic compound with a much higher purity is required.
As a purifying method of an organic compound, a zone melding technique is known. This technique is suitable for enhancing the purity of a reagent and concentrating a trace amount of impurity material. In principle, this technique utilizes the fact that when a melted material is subjected to normal freezing, there is a distribution (segregation) in its purity. More specifically, as shown in FIG. 10, a bar-shaped solid sample 1001 is melted into a band-shaped melted portion (melted zone) 1003 by a heater 1002, and the heater 1002 is moved slowly from one end of the sample to the other end, thereby purifying the sample.
Along with the movement of the heater 1002, liquid is frozen on the backward side (region a: 1004) of the melted zone, and a solid is melted on the forward side (region b: 1005). At this time, the concentration of an impurity is different between the solid portion and the liquid portion, which is used for purification.
FIG. 11 shows a portion close to A in a binary phase diagram of a solid solution containing a component A and an impurity component B (a portion close to the left end of the phase diagram corresponds to a sample melted into a band shape since the sample is substantially pure). A solid of an impurity (concentration CSO) starts precipitating at a solidification starting temperature TF. A concentration ratio represented by Cs/Cl is referred to as an equilibrium distribution coefficient KD, and K=Cs/Co is referred to as an effective distribution coefficient. At the beginning of solidification, C1=C0, so that KD=K. Actually, a concentration polarization occurs at an interface of a phase I (1006), a phase II (1003), and a phase III (1007), that is, a region a (1004) and a region b (1005), whereby K takes a value between 1 and KD.
In FIG. 11A, KD is 1 or less. However, in FIG. 11B, KD becomes 1 or more. Thus, the concentration of an impurity in a liquid or a solid increases. At a time of KD=1, a purification effect is not exhibited any more.
In other words, when KD is less than 1, a solid portion has a high purity. When KD is more than 1, a liquid portion has a high purity.
The purity of an organic compound can be enhanced by further purifying it before evaporation by utilizing the above-mentioned principle. However, even if an organic compound is purified, the purity thereof may be lowered before the organic compound is provided at an evaporation source in a film forming chamber.